Glaucoma is a common cause of blindness or visual impairment throughout the world, affecting about 1 person in 40 at some point in their life to a varying degree. In glaucoma, the fluid drainage pathways from the eye become restricted or damaged. As a result, fluid pressure within the eye (the intra-ocular fluid pressure, IOP) rises and the optic nerve is damaged by the raised fluid pressure.
Medical treatment, typically by the application of eye drops containing drugs which either decrease intraocular fluid production or increase outflow, requires lifelong compliance which is only infrequently attained in practice. Medical treatment is often only partially successful in retarding disease progression, with continued optic nerve damage and visual field loss.
The alternative is glaucoma drainage or filtration surgery (GFS), in which a drainage channel (the GFS channel or fistula) is created through the wall of the sclerocorneal junction by a laser beam or other means to connect the anterior chamber (the fluid space within the eyeball in front of the pupil) directly with the subconjunctival space (the space beneath the membrane covering the white of the eye). The GFS channel thus provides a route through which fluid can drain from the interior of the eye to the subconjunctival space. Successful GFS has been shown to provide better quality IOP control and better protection from disease progression than medical treatment in glaucoma.
However, problems arise in controlling drainage of fluid after GFS. Excessive drainage leading to hypotony (IOP less than a safe minimum level, which is about 5 mmHg) can result in a number of sight threatening complications including suprachoroidal haemorrhage and hypotony maculopathy. On the other hand, inadequate drainage with an insufficient IOP reduction (more than 15 mmHg) increases the risk of continued optic nerve damage. The IOP should ideally stay within the safe range (ie 5-15 mmHg) after GFS avoiding both early hypotony and later inadequate drainage. No current GFS technique achieves this goal consistently and complications after GFS remain common.
After GFS, limited subconjunctival scarring tends to form a partial seal around the area of the surgical drainage outlet (bleb formation). This process adds an additional flow resistance element (bleb resistance) and results in an increase in IOP from the initial postoperative level, which continues until postoperative inflammation subsides and the wound healing response is complete. This postoperative wound healing response varies considerably between individuals and is often modulated with anti-scarring treatments such as 5-fluorouracil and mitomycin C. IOP thus stabilises at a variable final level. To achieve the lowest safe final IOP level safely, current GFS techniques attempt to build a resistance element into the GFS channel (guarding, or fistular resistance) to protect from early overdrainage which can ideally be retained if bleb resistance is low, or abolished if bleb resistance is adequate to prevent late hypotony.
Currently preferred surgical practice, trabeculectomy with releasable sutures, uses a partial thickness scleral flap secured by one or more sutures tied in a slip knot to provide a trap-door type covering over the drainage passage through the wall of the eye. Guarding provided by this scleral flap, and hence the initial postoperative IOP level, is inconsistent. Also, early suture release often results in hypotony, whereas scarring beneath the scleral flap tends to make later suture release ineffective in abolishing fistular resistance.